1. Field of the Invention
The invention relates to a plate cylinder for a sleeve-type printform, particularly an offset printform at least partially made of aluminum, with a ramp-like phase piece as a guide surface for the mechanical expansion of the sleeve-type printform onto the larger cylinder diameter.
2. Description of the Prior Art
A plate cylinder of this type without channel and span segments, which can be used together with a sleeve-type a printform, is already known, for example, from German patent DE 41 06 062 C1. In order to place the printform onto the plate cylinder, the printform must be precentered on an end face of the plate cylinder. The sleeve-type printform is subsequently slipped on to the plate cylinder while being expanded by pressurized air. Stopping the pressurized air causes the sleeve to be placed on the cylinder in a form-fitting fashion, in the manner of a shrinkage fit. The sleeve can be removed from the cylinder by resuming the supply of pressurized air. The body of the cylinder preferably has a chrome coating to protect against corrosion.
In a manner which has been used for years in flexographic printing with galvanically-produced nickel sleeves, the sleeve, the inner diameter of which is smaller than the cylinder diameter of the carrying part, is expanded after precentering by means of a ramp-like transition piece of the cylinder body, so that the sleeve can be slipped completely onto the body of the cylinder over an air cushion.
The type of printform used most often in offset printing is the precoated aluminum plate, the great advantage of which is the simplicity and rapidity of its production. A sleeve-type printform which is made from an aluminum plate is known from the German patent application P 41 40 768.7. However, slipping the sleeve-type aluminum printform onto the coated cylinder can cause abrasion to the printform, which in turn may effect print quality.
The two materials used, respectively, for the printform and for the corrosion protection of the plate cylinder should both be retained, since aluminum is currently the established material for printforms and since chrome has proved itself preferable as protection against corrosion from current printing media.